Yet the size of the group and the type and strength of the social bonds, as well as location and behavior, vary considerably by species. In general, the largest groups are among the oceanic dolphins, such as spotted and common dolphins, which gather in groups of up to 500 in the eastern tropical Pacific. Melon-headed whales often associate in groups of 150 to 1,500, and striped dolphins sometimes travel in groups of 3,000. But typical numbers in most parts of the world, even of these species, are in the dozens. Many of these herds contain both sexes and all ages, but some species are further divided into adults of both sexes; females and calves; and immature animals of both sexes. Sometimes, male adults swim alone or in a separate group.
River dolphins and porpoises form the smallest groups. Often solitary, river dolphins rarely gather in herds of more than 10 to 15 individuals. And most true porpoises are found in groups of fewer than 10. Dall's porpoises, however, sometimes travel in schools of up to 3,00. Dolphins hunt in large numbers because it gives them an advantage in cornering or corralling schools of fish. Sometimes, several species of dolphins hunt side by side, but it is difficult to determine whether they are feeding separately or actually coordinating hunting maneuvers. Obviously, the more dolphins there are in a group, the more fish they will need to catch in order to make the reward worth sharing.
Some species have complex association. An individual orca, for example, might belong to many different groups at the same time. The resident orcas of British Columbia and Washington State are born into a maternal group, made up of brothers and sisters of all ages led by their mothers. One or more maternal groups make up a subpod. In subpods, the breeding females are usually sisters and the youngsters are siblings or first cousins. Subpods may travel independently for days or even months, but generally, they travel with other subpods as a pod.
Each pod has its own dialect, with some sounds unique to the pod. Pods that share some sounds are called clans. There are four resident clans off British Columbia and Washington State, three that make up the northern community and one the southern community. A community is composed of pods that associate from time to time, often for parts of every day during the summer. Two or more pods from a community travel together as a superpod.
Is an orca community equivalent to a population, or breeding unit? Because orca communities do not mix, they would each appear to represent a population. However, recent genetic studies have revealed that the northern and southern communities were probably once a single community and still form only one population.
Scientists can count herds and observe obvious behavior, such as different methods of hunting. But there are other ways they learn about the intimate details of social behavior. In the past, scientists often killed animals to study them, and many valuable things were learned this way. For the most part, though, it is no longer acceptable to kill wild animals to study them. Yet all species suffer natural and, from time to time, accidental deaths, and it is important that scientists use such opportunities to learn what they can.
When an entire group strands and dies on a beach, for instance, it is then possible to establish the sexes and ages in the group. through dissections, biologists can assess the fitness of the animals and perhaps discover what killed them. The number of times each female has ovulated can be determined by studying the reproductive organs. Layers in the teeth of some species can be counted to calculate the age.
Many things, however, can be learned only by patiently watching and monitoring the behavior and movement of live animals. the revolution in such study began in the 1970s, thanks to new photographic techniques, in particular, and to radio-tagging and satellite-monitoring technology. Individual animals and groups can now be followed from birth to death. The full results of this work are very slow in coming, but the rewards are great and are responsible for much of the excitement in dolphin and whale research today.
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